A scanning probe microscope is known as a measurement apparatus enabling observation of fine objects of the atomic order or size. In recent years, scanning probe microscopes have been applied to various fields such as measurement of the fine uneven shapes or inequalities on the surface of a substrate or wafer on which semiconductor devices are produced. There are various types of scanning probe microscopes depending on the detected physical quantity used for measurement. For example, there are scanning tunnel microscopes using tunnel current, atomic force microscopes using atomic force, magnetic force microscopes using magnetic force, etc. These are also expanding in ranges of application.
Among these, atomic force microscopes are sutiable for detecting fine uneven shapes of sample surfaces by a high resolution and are proving themselves in the fields of semiconductor device substrates, disks, etc. Recently, they are also being used for applications of in-line automatic inspection processes.
An atomic force microscope is basically comprised of a measurement device utilizing the principle of an atomic force microscope. Usually, it is provided with a tripod type or tube-type XYZ-fine movement mechanism formed using piezoelectric devices. At the bottom end of this XYZ-fine movement mechanism is provided a cantilever formed with a probe tip at its front end. The front end of the probe tip faces the surface of the sample. For example, an optical lever type optical detection device is provided for the cantilever. That is, a laser beam emitted from a laser light source (laser) arranged above the cantilever is reflected at the back surface of the cantilever and detected by a photodetector. If torsion or flexing occurs at the cantilever, the position of the photodetector which the laser beam strikes changes. Therefore, if displacement occurs at the probe tip and cantilever, the detection signal output from the photodetector can be used to detect the direction and amount of the displacement. The atomic force microscope usually is provided with a comparator and controller as a control system. The comparator compares a detection voltage signal output from the photodetector and a reference voltage and outputs a difference signal. The controller generates a control signal so that this difference signal becomes 0 and gives this control signal to a Z-fine movement mechanism in an XYZ-fine movement mechanism. In this way, a feedback servo control system is formed for holding the distance between the sample and probe tip constant. The mechanism can be used to make the probe tip scan the sample surface while following along its fine uneven and measure their shapes.
An atomic force microscope had as its central challenge the measurement of surface roughness of the “nm (nanometer)” size or less utilizing its high resolution at the time it was invented. However, at the present time, when its use has grown to in-line inspection in the semiconductor production process etc., measurement of extremely steep uneven shapes has become necessary. Further, automatic measurement of 90 degree vertical side parts, side walls, etc. at uneven shapes of the sample surface, which had been considered impossible in-line applications, is being sought.
As prior art for measuring such uneven surfaces, there is the surface profile inspection system disclosed in Japanese Patent Publication (A) No. 6-82248 (Japanese Patent No. 2501282). In this surface profile inspection system, the front end of a cantilever of the atomic force microscope is given dither motion by different frequencies in a direction substantially parallel to the axial direction of the front end (vertical axis) and a direction substantially perpendicular to the axial direction of the front end. By observing the movement of the front end at this time, it is possible to measure the distance between the sample surface and front end and the local gradient of the sample surface. The path of the scan movement of the front end is controlled by measuring local gradients as they are encountered. Further, the measured values are used to determine new scan positions and new feedback adjustment directions.
According to the above conventional system, it is necessary to give the dither motion to the part of the front end of the atomic force microscope, so the precision of estimation of the gradient at the point where edges, angles, etc. of the sample surface rapidly change in gradient drops and the reliability with respect to the measurement accuracy as a whole falls as a result. Further, since the front end is made to move along the sample surface based on the data of the gradient, precision of feed control also becomes necessary for all of the three-dimensional directions and control of movement of the front end becomes complicated.
[Patent Document 1] Japanese Patent Publication (A) No. 6-82248 (Japanese Patent No. 2501282)